This invention relates to a borehole logging method for measuring the die-away of nuclear radiation of a subsurface formation and for correcting signals representative of the decay constants and amplitudes of the measured radiation for the effects of detector standoff from the borehole wall of the formation where such signals may vary with time, distance, or any other independent variable. A porosity versus depth log is then produced for the subsurface formation as a function of the standoff corrected formation decay constant derived from calibrated measurements in borehole models of known porosities and conditions of detector standoff.
A borehole logging tool is conventionally used to measure the die-away of nuclear radiation in the formations surrounding a borehole. In neutron porosity logging, for example, a neutron source is utilized for bombarding the formations with fast neutrons as the logging tool is advanced through the borehole. In the course of moderation, the fast neutrons reach the epithermal range and thence are further moderated until they reach the thermal neutron range. The populations of neutrons at the various energy levels die-away with time following primary irradiation and thus offer means of characterizing the formations. The rate of die-away of the epithermal neutron population gives a quantitative indication of the amount of hydrogenous material present in the formations which in turn is indicative of the porosities of the formations. Examples of both methods and apparatus for carrying out epithermal die-away porosity logging are set forth in U.S. Pat. No. 4,097,737 to W. R. Mills, Jr., and U.S. Pat. Nos. 4,556,793 and 4,590,370 to L. S. Allen and W. R. Mills, Jr.
An article entitled "Improved Methods of Signal Processing For Pulsed Neutron Capture Logging", SPWLA Twenty Second Annual Logging Symposium, June 23-26, 1981 by R. Randall and E. C. Hopkinson discusses a method of pulsed neutron capture logging to differentiate oil, gas and saline formation water environments through casing. The method applies statistical averaging to a single exponential die-away term in the logged data to determine thermal neutron decay rate. In a still further reference, U.S. Pat. No. 4,600,838 to D. K. Steinman and L. A. Jacobson, there is described a method of thermal neutron die-away logging for overcoming excessive statistical fluctuations in the logged data, particularly in strongly absorbing formations. This method involves the determination of zero and first order moments of time during first and second discrete sequences of time gates respectively and the taking of the ratio of such moments to obtain a thermal neutron decay constant for the formations surrounding the logged borehole. In yet further references, Smith, Jr. U.S. Pat. No. 4,625,110, and U.S. Pat. No. 4,638,161 to Smith, Jr. and Verbout, there is described a porosity determination utilizing a two-exponential model approach to epithermal neutron die-away. Finally, Loomis U.S. Pat. No. 4,972,082 discloses use of a modulated exponential function to provide correction of an epithermal neutron die-away measurement for the situation where the logging tool is not in good contact with the borehole wall.
While the foregoing described methods and systems have been utilized in the production of well logs for characterizing the die-away of nuclear radiation of subsurface formations, there is still need for a very sensitive method of improving on the characterizations provided by porosity logs when the neutron detector of the logging tool is not in contact with the formation (i.e., separation or standoff of the logging tool detector from the borehole wall). Hereinafter, such separations will be referred to as standoff, although they can arise from the logging tool pulling away from the surface of the borehole wall or from the borehole enlarging away from the logging tool. In such cases, the formation decay constant determined from the die-away of nuclear radiation measurements will be in error by the degree of effect such standoff has on the measurement. It is therefore an object of the present invention to provide for a porosity log generated from a standoff corrected formation decay constant such that the formation may be accurately characterized as to porosity.